MSI doesn't handle empty substitution patterns correctly

EPICS 3.15.6 doesn't handle empty substitution patterns correctly anymore:

file "a.db" {
  { }
}

results in

msi: Parse error, unexpected '}'

No problems with older versions of Base. I'm able to work around the problem by using:

file "a.db" {
  { DUMMY="" }
}

This regression was introduced by the following commit:

https://git.launchpad.net/epics-base/commit/?h=3.15&id=67e2b74758982fd46be5529e28fcb93fd7eb59ad